1. Field of the Invention
The present invention relates to opaque quartz glass, particularly to opaque quartz glass which is excellent in heat insulation, and adhesiveness to transparent quartz glass. The present invention relates also to a process for producing the quartz glass.
2. Description of the Related Art
Conventionally, opaque quartz glass is produced by heat-fusion of powdery silicate as the source material for vitrification. The process of the heat-fusion includes a Bernoulli process in which the source material is fused in a flame such as an argon-oxygen plasma flame, and oxyhydrogen flame; and a vacuum fusion process in which silica packed in a container is heated and fused under a high vacuum. The opaque quartz glass is produced conventionally from natural silica rock or low-quality quartz as the source material. Such source materials contain many fine air bubbles which remain in the fused matter to give opaque quartz glass. In recent years, with the progress in integration of LSI in the semiconductor industry, the requirement for higher purity of the employed source material is becoming severer, and high-purity source materials have come to be used even in the fields in which the use of low-purity source materials has been conventional. A typical example is a flange material, for which opaque and pure quartz glass, namely high-purity opaque quartz glass, is demanded. However, the conventional natural source material for opaque quartz glass contains a large amount of impurities together with the fine bubbles. The removal of the impurities from the source material is extremely difficult, and refinement of the source material for high purity is considered to be impossible. On the other hand, quartz of relatively high purity, which contains less air bubbles, in particular, fine air bubbles in a smaller amount, does not increase its opacity on fusion and produces translucent quartz glass.
An improved process therefor is disclosed in which high-purity amorphous silica containing a lesser amount of elements of alkali metals, alkaline earth metals, Fe, and Al, containing many fine air bubbles, and containing silanol groups as a gasifiable component uniformly in a specified concentration range is fused in a flame (JP-A-6-24771). This process is applicable only to production of quartz glass products in a simple shape such as a base ingot for an IC-sealing silica filler and for silica glass powder, but is not suitable for production of quartz glass products in a complicated shape such as flanges, cylinders, hollow square prisms, screws, turbo-chargers, a cubic water container or a wash basin because of the necessity of post-machining such as cutting in a larger amount, resulting low utilization efficiency of the quartz glass and rise of the production cost.
Another process for opaque quartz glass is disclosed in which highly purified powdery crystalline quartz is ammoniated by heating in an ammonia atmosphere and fusing in an inert gas atmosphere to incorporate a larger number of smaller bubbles to increase the total bubble sectional area and to improve heat resistance (JP-A-7-61827, and JP-A-7-300341). However, in this process, the density, bubble diameter, and the amount of the bubbles depend greatly on the particle size, the particle size distribution, and the packing state of the powdery source material, so that the bubble diameter cannot readily be controlled with reproducibility, and the bubble diameter and the bubble amount vary significantly from the surface to the interior disadvantageously. Further, the produced quartz glass contains nitrogen element at a high content. This nitrogen will be liberated from the glass substrate in a process of repairing of defects of the quartz glass article by flame working, causing undesirable change of the dimension or the appearance such as surface roughness and opacity change at the repaired portion. Furthermore, this opaque quartz glass cannot be joined to a transparent quartz glass member by flame welding because of large difference in the nitrogen element content.
Other processes for opaque quartz glass are disclosed in which fine powder of a material such as carbon and silicon nitride is added as a blowing agent to the silica source material such as silica rock, silica sand, .alpha.-quartz, and cristobalite (e.g., JP-A-4-65328). However, such processes are not suitable for semiconductor manufacturing jigs or the like used at a high temperature for long time, because OH groups can be incorporated into the glass in fusion by oxyhydrogen flame, resulting in a lower viscosity at high temperatures, although the disadvantages of the aforementioned processes are offset. Moreover, in these processes, uniform dispersion of the fine gas bubbles in the fusion is not readily achievable since the process includes mixing of solid particles and reactions or decomposition in a solid phase. In the flame-fusion process, since the fine particles stay in the flame for an extremely short residence time, the reaction tends to be incomplete, and the added blowing agent may remain as an impurity in the fused matter, or the fused matter may be colored by reaction of the silicate source material with the blowing agent.
As discussed above, any of the aforementioned prior art involve various unsolved problems.
The present invention has been accomplished to solve the above problems of the prior art.